Originally Posted by biniou
(and in fact, --max-depth=1 won't give an actual value, as it stops at directory depth of 1 ...)
As I understand it, the man page says the --max-depth argument does not
affect how far down du traverses to count used space, but only affects the depth of the final report. That is to say '--max-depth=1' will cause du's final report to display only the directory specified on the command line and one level of directories below it. Each directory reported will have an accurate accounting of the space consumed by that directory and all its subdirectories.
The df and du utilities look at different data to determine disk usage. In fact, I've seen this question, and variations of it, asked a number of times, but I can't find another thread at the moment. However, searching on Google for "df du different" gave this result on the first page: DF & DU report different size
And another support thread on the first page: "df" and "du" show different results
I don't know if they'll answer the question to your satisfaction, but they're a good start.